1. Field of the Invention
The present invention relates to an optical recording medium including a plurality of recording and reading layers, and an optical recording-reading method of recording on the optical recording medium.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, DVD-RAMs, Blu-ray Discs (BD) are widely used to view digital video contents and to record digital data. In the BD standard, which is one of the next generation DVD standards, the wavelength of the laser beam used for recording and reading is reduced to 405 nm, and the numerical aperture of an objective lens is set to 0.85. In optical recording media conforming to the BD standard, tracks are formed at 0.32 μm pitch. This enables 25 GB or more data to be recorded on and read from one recording and reading layer of such an optical recording medium.
It is expected that the size of video and data files will increase more and more in the future. Therefore, it is contemplated to increase the capacities of optical recording media by using a multiple stack of recording and reading layers. In the technologies for BD standard optical recording media reported by I. Ichimura et al., Appl. Opt., 45, 1974-1803 (2006) and K. Mishima et al., Proc. of SPIE, 6282, 628201 (2006), 6 to 8 recording and reading layers are provided to achieve ultra large capacities as much as 200 GB.
When a multiple stack of recording and reading layers is used in an optical recording medium, projections and depressions, such as grooves and lands, for tracking control are formed on each recording and reading layer, and therefore a stamper (a master die) must be used to form the projections and depressions each time a recording and reading layer is formed. Therefore, as the number of stacked layers increases, the number of times the stamper is used increases, and this may result in an increase in manufacturing cost.
In the technologies for optical recording media recently proposed in Japanese Patent Application Laid-Open Nos. 2002-63738, 2008-97694, and 2009-104717, a servo layer is provided separately from recording and reading layers, and information is recorded on each recording and reading layer using a recording laser beam while a tracking signal is obtained from the servo layer using a servo laser beam. With such technologies, the recording and reading layers need not have projections and depressions (grooves) for tracking information, and a stamper need not be used for the recording and reading layers during manufacturing. This enables a significant reduction in cost.
One problem with the optical recording media described in Japanese Patent Application Laid-Open Nos. 2002-63738, 2008-97694, and 2009-104717 is that, to increase the recording capacity of the recording and reading layers, the track pitch of each servo layer must be reduced.
For example, in the structure of the optical recording medium in Japanese Patent Application Laid-Open No. 2008-97694, two laser beams of the same type are used to irradiate a servo layer and recording and reading layers, respectively, and accordingly, the track pitch of the servo layer coincides with the track pitch of the recording and reading layers. However, this causes the problem in that the recording capacity of the recording and reading layers is restricted by the track pitch of the servo layer.
For example, in the optical recording medium described in Japanese Patent Application Laid-Open No. 2009-104717, the wavelength of the laser beam used to irradiate a servo layer is different from the wavelength of the laser beam used to irradiate recording and reading layers. The recording capacity of the recording and reading layers is restricted by the larger one of the spots projected onto the servo layer and the recording and reading layers. Therefore, although the laser beams with different wavelengths are used, the optical systems in an optical pickup are designed such that the spots of the laser beams have the same diameter. However, the problem still remains that the recording capacity of the recording and reading layers is restricted by the track pitch of the servo layer.